Safe Port/Stream from Cell

proc {NewSend List Send}
   C={NewCell _}
in
   proc {Send Msg}
      Tail
   in
      {Exchange C Msg|!!Tail Tail}
   end
   List=!!{Access C}
end

• the free variable Tail is only in the cell
• the previous tail is instantiated with Msg|!!Tail, thus consumers only see the read-only version of Tail